Surgical kit for hemiarthroplasty hip replacement

ABSTRACT

The present invention concerns apparatus and methods for hip refurbishment/replacement and in one aspect provides a kit comprising a femoral head and reamer. The present invention also provides apparatus and methods for minimally invasive surgery on a hip or similar (eg shoulder) joint. The kit is suited for performing a hip hemiarthroplasty, in which a femoral head is fitted directly into a socket reamed into the acetabular without any permanent liner or prosthetic acetabular being implanted. The reamer is used to ream out the acetabulum until cancellous bone is exposed so that it bleeds liquid containing stem cells. By selecting the size of the femoral head in accordance with the characteristics of the patient, the pressure imposed on the liquid/synovial fluid of the joint is in the range between 0.5 and 2 MPa This causes the stem cells to produce new cartilage between the bone and femoral head.

FIELD Of THE Invention

The present invention concerns apparatus and methods for hiprefurbishment/replacement and in one aspect provides a kit comprising afemoral head and reamer. The present invention also provides apparatusand methods for minimally invasive surgery on a hip or possibly even ona shoulder or other joint.

BACKGROUND OF THE INVENTION

In conventional hip replacement surgery the surgeon replaces the femoralhead and reams out the acetabulum prior to insertion of a prostheticacetabulum. The prosthetic femoral head, stem, acetabulum and reamer areall provided as a kit by an implant supplier, such as the applicant, inaccordance with a set of patient specification determined by thesurgeon. The reamer is designed to ream out exactly sufficient materialfrom the acetabulum to fit the replacement prosthetic acetabulum. Thisform of hip replacement is known as total hip replacement in some casesthe hip is repaired by hemiarthroplasty in which the femoral head onlyis replaced. In such cases the natural femoral head is replaced in muchthe same way as with total hip replacement, however, the acetabulum isleft substantially untouched with the natural cartilage still in place.Therefore the surgical kit for hemiarthroplasty does not include areamer or acetabulum.

The prosthetic acetabulum may be said to have a male side which isclosely received into the socket formed by the reamer and a female sidewhich provides the socket into which the prosthetic femoral headdirectly and closely fits. The reamer is therefore sized to complementthe male side of the prosthetic acetabulum.

In use the prosthetic acetabulum provides the bearing surface for thefemoral head. However, being inert, the bearing surfaces of theacetabulum wear during use until the replacement hip joint needs repair.It is desirable to defer or avoid this problem completely because therepair process involves a major surgical procedure with the associateddiscomfort and risk to the patient.

The present invention aims, amongst other objectives, to alleviate thisproblem.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided asurgical kit for hip replacement comprising:

a prosthetic femoral head and a reamer,

said reamer being adapted to ream a socket into an acetabulum until thecancellous bone is exposed,

the femoral head having a size and shape complementary to the reamersuch that the femoral head can be fitted closely and directly into areamed acetabulum whereby liquid between the femoral head and the socketwill be subjected to a hydrostatic pressure in the range of 0.01-5 Mpa.

The steps of the surgical procedure comprise first identifying certaincharacteristics of the patient which may include their weight and thedimensions of the natural femur and pelvis. Using known methods such asthose developed by G. Bergman, the implant manufacturer determines thesize and shape (especially radius of curvature) of femoral head requiredwhich will result in any liquid between the femoral head and a closelymatching socket being subject to a pressure of between 0.01 and 5 MPa.The femoral head and matching reamer are then produced for the kit.

Detailed descriptions of the Bergman methodology, which entailradiographic assessment of the contact area of the joint and adjustingfor weight and level of activity of a patient to estimate an averagefigure for the force across the joint are given In the following threearticles, the contents of which are incorporated herein by reference: 1) Bergmann G, et al. Hip contact forces and gait patterns fromroutine activities. J Biomech. 2001 July;34(7):859-71. 2) Graichen F,Bergmann G, Rohimann A. Hip endoprosthesis for in vivo measurement ofjoint force and temperature. J Biomech. 1999 October;32(10):1113-7; and3) Bergmann G, Graichen F, Rohlmann A. Hip joint loading during walkingand running, measured in two patients. J Biomech. 1993August;26(8):969-90. Review.

The dimensions of the reamer are chosen to closely match the dimensionsof the femoral head. Generally the radius of curvature of the reamer (iecutting envelope of the reamer) will be at most approximately 5 mmgreater than that of the femoral head so that the clearance between thefemoral head and acetabulum is 5 mm or less. This is broadly less thanhalf that of a conventional total hip replacement, where an acetabularshell is inserted in the hip to serve as the bearing surface. Indeed Inthe preferred embodiments the radius of curvature of the reamer will beat most approximately 3 mm greater than that of the femoral head so thatthe clearance between the femoral head and acetabulum is 3 mm or lessand may be only 1 mm to 2 mm.

In an alternative definition of the invention there is provided asurgical kit for hip replacement comprising:

a prosthetic femoral head and a reamer,

said reamer being adapted to ream a socket into an acetabulum until thecancellous bone is exposed,

the femoral head having a size and shape complementary to the reamersuch that the femoral head can be fitted closely and directly into areamed acetabulum, the size (radius of curvature) of the reamer (iecutting envelope of the reamer) being at most approximately 5 mm greaterthan that of the femoral head so that the clearance between the femoralhead and acetabulum is 5 mm or less.

In the surgical procedure the reamer supplied in the kit is used by thesurgeon to ream out the cartilage lining the acetabulum and then thecortical bone until the underlying cancellous bone is exposed andbleeding. The liquid bleeding from the cortical bone includes stemcells. The femoral head is secured to a femoral stem socketed into thepatient's natural femur in a substantially conventional way and thefemoral head socketed into the reamed acetabulum.

By selecting the size of the femoral head in accordance with thecharacteristics of the patient, the liquid, including synovial/jointfluid, is thus subject to a hydrostatic pressure in the range of 0.01-5MPa., preferably in the range of 0.5-2MPa and more preferably near to 2MPa. The stem cells within the liquid are, as a result of the pressure,encouraged to form chondrocytes which grow into cartilage lying on thesubchondrial bone which provides a natural active bearing surface forthe femoral head, rather than relying on a rigid wholely prostheticbearing comprising a rigid acetabular shell prosthesis as used in totaljoint replacement, and giving a better refined joint than ahemiarthroplasty .

In order to further promote the formation of cartilage spacers may beprovided to separate the surface of the femoral head and the reamedacetabulum. The spacers may be provided on the surface of the prostheticfemoral head but preferably are mounted to the acetabular surface, andmay be formed of material which is resorbable to provide a temporarybearing surface which is resorbed as the cartilage develops. Suitableexample materials from which the spacers may be formed to. be resorbableare diverse and numerous. Examples include fibronectin, PLGA andpolyfumarate

In a further development the kit may further include a membrane whichsuitably conforms to the shape of the surface of the femoral head andthe reamed acetabulum. The use of a continuous/contiguous membrane asopposed to discrete spacer elements provides a more uniform and widelyspread transfer of loading between the femoral head and the acetabulum.The membrane preferably also is adapted to be resorbable or is adaptedto be removable from the joint once sufficient regeneration of cartilagehas occurred. Preferably the membrane is formed in situ and suitably iscomposed of a gel/hydrogel with or without fibre reinforcement. Examplefibre reinforcements may comprise polyester or fibronectin or otherfibrous materials. Resorbable fibres such as fibronectin are preferred.Fibrin or fibrin glue may be used.

Other preferred compositions of the membrane include cellulose nitrate,expanded PTFE, Dacron, alginate and glycolic acid-lactic acid complex(PLGA). Polyurethane may be used on its own or with other materials andmay, for example, be used in the form of a foam. Another preferredmaterial is a collagen mesh or gel. Again, fibronectin or polyfumarateare other preferred materials usable and suited to resorbton.

The material or materials from which the femoral head surface, membraneand or spacers are formed are suitably compliant, suitably havingelastohydrodynamic deformation properties, with different moduli, andsuitably having different frictional characteristics to control thestresses and friction on the reamed bone surface. Particularlypreferably it is the external surface of the femoral head that iscompliant/pliant and as an example it may comprise a layer ofpolyurethane. The spacers suitably are relatively more rigid and atleast more rigid than any corresponding membrane.

In those embodiments having a membrane or spacers, the membrane orspacers suitably are porous or permeable to facilitate delivery ofgrowth factors, stem cells, chondrocytes or fibroblaststherethrough/therefrom to encourage the formation of cartilage. Examplegrowth factors that might be used include transforming growth factorbeta, insulin-like growth factor and other known cartilage growthfactors. Platelet derived growth factor (PVGF) and fibroblast growthfactor (FGF) may usefully be used. The growth factors, stem cells,chondrocytes or fibroblasts may suitably be bound to the spacers ormembrane and released as the lafter degrade.

Where a membrane is used this may suitably be cemented in place on thefemoral head and/or on the acetabular surface. The membrane may beprovided with ribs or other reinforcements to counter any rucking/shearof the membrane in use. As noted above, the membrane preferably isformed in situ and suitably absorbs water for deployment. Anyreinforcements may be expandable/formed in situ to facilitate deploymentof the membrane using a non-invasive/minimally-invasive surgicalprocedure.

In a further aspect of the present invention, there is provided anon-invasive reaming procedure for joint refurbishment and which isparticularly useful where neither the head of the femur nor theacetabular surface needs to be fully replaced.

In the procedure an access tunnel is first drilled through the femoralhead and neck and a minimal incision is made to the (hip) joint capsule.A modular shell reamer having a separable substantially part-sphericalhead and shaft can be assembled in situ by introducing the shaft of thereamer through the tunnel, introducing the reamer head through the slitin the capsule and coupling the inserted end of the reamer shaft to thereamer head in situ. By use of a substantially unique double-sidedreamer head, having reamer cutting teeth facing not only outwardlytoward the acetabular surface but also inwardly toward the femoral headsurface, both of those two surfaces may be simultaneously reamed to bethoroughly compatible with each other. The acetabular surface may bereamed by pushing the reamer via the shaft against the acetabularsurface and conversely the femoral surface may be reamed by pulling thereamer shaft to force the surface of the reamer head facing the femoralsurface into contact therewith. The resulting femoral head andacetabular surfaces are congruent and they define therebetween therequired joint space. A membrane may be interposed between the tworeamed surfaces and is suitably of the form described previously, beingpermanent or temporary, restorable and/or resorbable and suitablyadapted to release stem cells, chondrocytes, growth factors or similarinto the joint space, if required.

The access tunnel may usefully serve as a conduit for delivery of amedium for forming the membrane in situ. A biologically active gel orgelling solution such as alginate or a hydrogel suitably containing acombination of stem cells, chondrocytes, growth factors or similar maybe injected into the joint space via the access tunnel. Alternatvely,the gel/medium for forming the membrane may be injected directly intothe capsule rather than via the access hole.

The minimally invasive reaming method may also be employed with a singlesided reaming head but to substantially less advantage.

The components of the femoral head may also include means to delivergrowth factors to encourage the formation of cartilage. The spacers maydeliver growth factors as they are resorbed.

BRIEF DESCRIPTION OF THE DRAWINGS

A surgical kit for hemiarthroplasty hip replacement embodying thepresent invention will now be described, by way of example only, withreference to the accompanying illustrative figures, in which:

FIG. 1 illustrates a femoral head;

FIG. 2 illustrates a reamer;

FIG. 3 is similar to FIG. 1 but illustrates the apparatus in use in ahemiarthroplasty and further illustrates the provision of a membranebetween the femoral head of the femoral prosthesis and the acetabulum.(FIG. 3A is a close-up of the interface between the femoral head,membrane and acetabulum);

FIG. 4A is a view similar to FIG. 3 but of a minimally-invasive surgicalprocedure to refurbish a hip joint when the femur is not replaced by afemoral prosthesis but where the femoral head surface and acetabularsurface are each reamed in situ;and

FIGS. 4B to 4D show successive stages of the surgical procedure.

DESCRIPTION OF THE PREFERRED EMODIMENT

FIG. 1 illustrates a prosthetic femoral head 1 which may be integralwith a femoral stem or adapted for fifting to a femoral stem as isknown. A femoral shaft 2 extends from the femoral head 1. The femoralhead 1 is part spherical and provided with spacers 3 located atintervals across its surface to provide a temporary bearing surface. Thespacers 3 are formed from a material which is gradually resorbed andgradually dispenses growth factors.

A reamer is shown in FIG. 2 and has a part spherical head 4. Abradingelements 5 of substantially conventional design are dispersed across thespherical surface of the reamer so that in use the reamer produces asocket in an acetabulum corresponding in size and shape to the envelopeindicated by the outer dotted line 6.

As can be seen in FIG. 1 the outer dotted line 6 also forms an envelopearound the bearing surface provided temporarily by the spacers 3 formedon the surface of the femoral head 1. The size of the femoral head 1 isdetermined via reference to the characteristics such as the weight ofthe patient so that the pressure of liquid present between the femoralhead and the socket formed In the acetabulum by the reamer is in therange between 0.01-5 MPa, and preferably close to 2 MPa, for example inthe range between 0.5-2 MPa.

The femoral head fits directly into the socket produced by the reamerwithout any Intervening, permanent hard liner. The material from whichthe surface of the femoral head is made may be compliant to control thehydrostatic pressure and suitably is of polyurethane.

Referring to FIG. 3, the apparatus is here shown in use in ahemiarthroplasty between the acetabulum 8 of a hip 7 and the shaft of afemur 9, but is further improved by the provision of a membrane 10between the femoral head 1 of the femoral prosthesis and the acetabulum8. The membrane is a thin pliable membrane formed of a hydrogel andhereshown enwrapping the prosthetic femoral head 1 to transfer the loadsacting across the joint more broadly and evenly than achieved by spacersalone.

Referring to FIGS. 4A to 4D, these show the successive stages of aminimally-invasive surgical procedure to refurbish a hip joint 7,9 wherethe head of femur 9 is not replaced by a femoral head prosthesis and theacetabulum 8 of the hip 7 is not lined with an acetabular shellprosthesis, but where the head surface of the femur and the acetabularsurface are each simply reamed in situ.

Here an access tunnel 11 is first drilled through the femur 9 head andneck and a minimal incision is made to the (hip) joint capsule. Amodular shell reamer having a separable substantially part-sphericalhead 13 and shaft 12 can be assembled in situ by introducing the shaft12 of the reamer through the tunnel 11, introducing the reamer head 13through the slit in the capsule and coupling the inserted end of thereamer shaft 12 to the reamer head 13 in situ.

By use of a substantially unique double-sided reamer head 13, havingreamer cutting teeth 5 facing not only outwardly toward the acetabularsurface 8 but also inwardly toward the femoral head surface opposing theacetabular surface, both of those two surfaces may be simultaneouslyreamed to be thoroughly compatible/congruent with each other. Theacetabular surface 8 may be reamed by pushing the reamer head 13 via theshaft 12 against the acetabular surface 8 and conversely the femoralsurface may be reamed by pulling the reamer shaft to force the surfaceof the reamer head facing the femoral surface into contact therewith.

On removal of the reamer (FIG. 4C) the resulting femoral head andacetabular surfaces are congruent and they define therebetween therequired joint space. A membrane 10 may be interposed between the tworeamed surfaces and is suitably of the form described previously, beingpermanent or temporary, restorable and/or resorbable and suitablyadapted to release stem cells, chondrocytes, growth factors or similarinto the joint space, if required.

The access tunnel 11 may usefully serve as a conduit for delivery of agel forming medium for forming the membrane 10 in situ. A biologicallyactive gel or gelling solution such as alginate or a hydrogel suitablycontaining a combination of stem cells, chondrocytes, growth factors orsimilar may be injected into the joint space via the tunnel 11 or elseinjected directly into the capsule via the slit/incision or otherwise.

Although described hereinabove in the context of a method and apparatusfor replacement/refurbishment of a hip joint, some or all of the aboveteachings may be applicable to other joints that have a broadly ball andsocket configuration.

1. A surgical kit for hip replacement comprising: a prosthetic femoralhead and a reamer, said reamer being adapted to ream a socket into anacetabulum until the cancellous bone is exposed, the femoral head havinga size and shape determined with reference to the weight of the patientfor whom the surgical kit is intended and complementary to the reamersuch that the femoral head can be fitted closely and directly into areamed acetabulum whereby liquid between the femoral head and the socketwill be subjected to a hydrostatic pressure in the range of 0.01-5 MPa.2. A surgical kit according to claim 1, wherein the hydrostatic pressureis in the range 0.5-2 MPa.
 3. A surgical kit according to claim 2,wherein the hydrostatic pressure is 2 MPa.
 4. A surgical kit accordingto claim 1 wherein a membrane is provided which in use spaces apart thesurface of the femoral head and the reamed acetabulum.
 5. A surgical kitaccording to claim 1, wherein multiple spacers are provided which in usespace apart the surface of the femoral head and the reamed acetabulum.6. A surgical kit according to claim 4, wherein the membrane is ofresorbable material.
 7. A surgical kit according to claim 5, wherein themultiple spacers are of resorbable material.
 8. A surgical kit accordingto claim 4, wherein the kit has a continuous/contiguous membrane withconforms to the shape of the surface of the femoral head and the reamedacetabulum.
 9. A surgical kit as claimed in claim 4, wherein the kit hasa membrane that is composed of a gel/hydrogel.
 10. A surgical kit asclaimed in claim 9, wherein the gel/hydrogel has fibrous materialstherein for reinforcement.
 11. A surgical kit as claimed in claim 4,wherein the kit has a membrane that comprises material selected from:celulose nitrate, expanded PTFE, dacron, alglnate and glycolicacid-lactic acid complex (PLGA); polyurethane; collagen mesh or gel;fibronectin; and polyfumarate.
 12. A surgical kit according to claim 1,wherein the surface of the femoral head is formed from a materialadapted to deform and so sustain the hydrostatic pressure.
 13. Asurgical kit as claimed in claim, wherein the kit comprises multiplespacers and a membrane and the membrane is less rigid than the spacers.14. A surgical kit according to claim 4 wherein the membrane is adaptedto deliver growth factors, stem cells, chondrocytes or fibroblasts tothe liquid.
 15. A surgical kit according to claim 5 wherein the spacersare adapted to deliver growth factors, stem cells, chondrocytes orfibroblasts to the liquid.
 16. A surgical kit as claimed in claim 14,wherein said membrane is porous or permeable.
 17. A surgical kit asclaimed in claim 15, wherein said spacers are porous or permeable.
 18. Asurgical kit according to claim 1, wherein the size (radius ofcurvature) of the reamer (ie cutting envelope of the reamer) is at mostapproximately 5 mm greater than that of the femoral head so that theclearance between the femoral head and acetabulum is 5 mm or less.
 19. Asurgical kit comprising a reamer wherein the reamer is a modular shellreamer for joint refurbishment of a ball and socket anatomical jointsuch as a hip joint, having a shaft and a substantially part-sphericalhead separable from the shaft but capable of being securely coupled tothe shaft in situ for use, the reamer head having reamer cutting teethfacing not only outwardly toward the socket surface in use but alsoinwardly toward the ball surface.
 20. A surgical kit for hip replacementcomprising: a prosthetic femoral head and a reamer, said reamer beingadapted to ream a socket into an acetabulum until the cancellous bone isexposed, the femoral head having a size and shape complementary to thereamer such that the femoral head can be fitted closely and directlyinto a reamed acetabulum, the size (radius of curvature) of the reamer(ie cutting envelope of the reamer) being at most approximately 5 mmgreater than that of the femoral head so that the clearance between thefemoral head and acetabulum is 5 mm or less.
 21. A surgical kitaccording to claim 20, wherein a membrane is provided which in usespaces apart the surface of the femoral head and the reamed acetabulum.22. A surgical kit according to claim 20, wherein multiple spacers areprovided which in use space apart the surface of the femoral head andthe reamed acetabulum.
 23. A surgical kit according to claim 20, whereinthe membrane is of resorbable material.
 24. A surgical kit according toclaim 20, wherein the multiple spacers are of resorbable material.
 25. Asurgical kit according to claim 20, wherein the kit has acontinuous/contiguous membrane with conforms to the shape of the surfaceof the femoral head and the reamed acetabulum.
 26. A surgical kit asclaimed in claim 20, wherein the surface of the femoral head is formedfrom a material adapted to deform and so sustain the hydrostaticpressure.
 27. A surgical kit as claimed in claim 20, wherein the kitcomprises multiple spacers and a membrane and the membrane is less rigidthan the spacers.
 28. A method for hemiarthroplasty of a hip jointcomprising providing a prosthetic femoral head and a reamer, and usingthe reamer to ream a socket into an acetabulum until the cancellous boneis exposed, the femoral head having a size and shape closelycomplementary to the reamer, and fitting the femoral head directly intothe reamed acetabulum, the configuration being such that liquid betweenthe femoral head and the socket will be subjected to a hydrostaticpressure in the range of 0.01-5 MP to stimulate formation of newcartilage between the bone and femoral head.
 29. A minimally-invasivereaming procedure for joint refurbishment of a ball and socketanatomical joint such a hip joint, comprising forming an access tunnelthrough the ball part of the joint, providing a modular shell reamerhaving a separable substantially part-spherical head and a shaft, andintroducing the shaft of the reamer through the tunnel, introducing thereamer head separately and coupling the inserted end of the reamer shaftto the reamer head in situ, the reamer head, having reamer cutting teethfacing not only outwardly toward the socket surface but also inwardlytoward the ball surface, and manipulating the reamer to ream both thesocket surface and the ball surface.
 30. A modular shell reamer forjoint refurbishment of a ball and socket anatomical joint such as a hipjoint, having a shaft and a substantially part-spherical head separablefrom the shaft but capable of being securely coupled to the shaft insitu for use, the reamer head having reamer cutting teeth facing notonly outwardly toward the socket surface in use but also inwardly towardthe ball surface.
 31. A method of making a surgical kit for hipreplacement, the kit being as claimed in claim 1, the method comprising:determining the weight of the patient whose hip is to be replaced andestimating the contact area of the patient's hip joint required toensure a hydrostatic pressure within the hip joint in the range of0.01-5 Mpa and providing a prosthetic femoral head and a reamer, thereamer being adapted to ream a socket into an acetabulum until thecancellous bone is exposed, the femoral head having a size and shapecomplementary to the reamer and selected to ensure the required contactarea such that the femoral head can be fitted closely and directly intoa reamed acetabulum of the patient's hip joint whereby liquid betweenthe femoral head and the socket will be subjected to a hydrostaticpressure in the range of 0.01-5 MPa.
 32. A surgical kit for hipreplacement comprising a reamer, said reamer being adapted to ream asocket into an acetabulum until the cancellous bone is exposed, the size(radius of curvature) of the reamer (ie cutting envelope of the reamer)being at most approximately 5 mm greater than that of a femoral head ofa patient on whom the reamer is to be used so that the clearance betweenthe femoral head and acetabulum is 5 mm or less.
 33. A surgical kitaccording to claim 32, wherein a membrane is provided which in usespaces apart the surface of the femoral head and the reamed acetabulum.34. A surgical kit according to claim 32, wherein multiple spacers areprovided which in use space apart the surface of the femoral head andthe reamed acetabulum.